Bisexuality is common among mammalian species. We are investigating the hypothesis that in nonprimate mammals the degree of bisexuality expressed in adulthood depends upon the actions of hormones both during adulthood and during early stages of development. In primates, although these same hormonal variables operate, their actions can be modified or overridden by experiential factors related to the conditions of rearing. Experiments are underway to determine the manner in which gonadal hormones act during fetal life to influence patterns of sexual and sex-related behaviors and to determine the manner in which these same chemical substances act to regulate the expression of these behaviors at later developmental ages. Biochemical methods have been developed for competitive protein binding and radioimmunoassay of a variety of gonadal hormones which permit quantitative measurement of steroids in peripheral venous blood or in endocrine gland venous effluent. In addition, methods are in use for the study of in vitro and in vivo metabolism and biotransformation of radio-labelled steroids in neural target tissues presumed to be involved in the mediation of behavior. Behavioral methods have been developed for studies of sexual and social behavior on several mammalian species. New methods for the analysis of motivational states utilizing operant techniques are in the process of development. Biochemical and behavioral studies are carried out concurrently on a variety of species. This attempt to develop information within the same laboratory on a broad comparative front is being made for what such studies will contribute to our understanding of related problems in man.